1. Field of the Invention
This invention relates to semiconductor memories and circuitry associated therewith. More particularly, this invention relates to sense amplifiers for determining the logic states of dynamic memory cells and to methods for sensing such logic states.
2. Description of the Prior Art
Several types of sense amplifiers have been suggested for determining the logic states of memory cells in metal-oxide semiconductor (MOS) dynamic memories. In U.S. Pat. No. 3,514,765, A. O. Christensen discloses a basic sense amplifier utilizing a pair of cross-coupled MOS field-effect transistors (FET's) which operate in a racing mode to sense either a binary low logic state or a binary high logic state in a dynamic memory cell. S. Wahlstrom discloses another such sense amplifier in U.S. Pat. No. 3,678,473. The sensitivity of a sense amplifier depends on the voltage difference supplied to the cross-coupled FET's. In the sense amplifiers of Christensen and Wahlstrom, this voltage difference is often so small as to cause errors. These prior art devices also do not include any means for restoring the logic state to the memory cell.
L. G. Heller et al. disclose a dynamic sense amplifier in which an initial voltage difference indicative of the logic state of a dynamic memory cell is pre-amplified before being sensed by a pair of cross-coupled FET's in "High Sensitivity Charge-Transfer Sense Amplifier," IEEE J. of Solid-state Circuits, vol. SC-11, no. 5, October 1976, pp. 596-601. The prior art device of Heller et al. provides high sensitivity and also restores the sensed logic state but, however, is extremely slow and requires multiple external power supplies making the device impractical.